1. Field
Embodiments of the present invention relate generally to oil film bearings designed to rotatably support the roll necks of rolls in a rolling mill, and are concerned in particular with spherical oil film bearings useful primarily although not exclusively in rotatably supporting the tapered roll necks of back-up rolls in mills producing flat products.
2. Description of Related Art
Rolling mills producing flat products generate extremely large forces which highly load the structural components of the mill stands. These highly loaded components undergo large deflections that must be accommodated. As shown diagrammatically in FIG. 1, of particular significance is the deflection of the back-up rolls 10 which provide support for the work rolls 12. The roll necks of the back-up rolls are typically supported in oil film bearings 14 of the type disclosed for example in U.S. Pat. No. 6,123,461 (Wojtkowski) and U.S. Pat. No. 6,468,194 (Wojtkowski et al.), the descriptions of which are herein incorporated by reference in their entirety. The bearings are contained in housings 16 commonly referred to as “chocks.”
To compensate for roll deflection, the chocks 16 must be able to tilt so that the oil film bearings 14 can remain aligned with the roll ends.
The current design practice is to allow the chocks to tilt by employing rocker plates 18 for the chocks of the bottom back-up rolls, and curved seats 20 for the chocks of the top back-up rolls. However, the rocker plates and curved seats are subject to wear leading to failure, causing problems that affect both the bearings 14 and the ability of the mill to roll precision products.
In an earlier effort at addressing this problem, and as disclosed in U.S. Pat. No. 2,312,648 (Jones), it has been proposed to replace the conventional oil film bearings with spherical oil film bearings. With reference to FIG. 2, it will be seen that spherical oil film bearings 22 are capable of accommodating roll deflection within the confines of their respective chocks 24, thus eliminating the need to resort to chock tilting by employing associated rocker plates and curved seats. Elimination of these components simplifies mill design, increases mill stiffness and reduces the total height of the roll stack.
Notwithstanding the potential advantages of employing spherical oil film bearings in highly stressed rolling mills, and despite its debut over seventy years ago with the issuance of the Jones patent, it appears that this technology has never been employed in heavily loaded rolling mill applications. A number of drawbacks have contributed to this delay, including, to name a few, a discontinuous internal spherical bearing surface provided by separated multiple bushing components which are urged apart against internal chock surfaces, making them difficult to assemble and maintain in alignment within required tolerances when subjected to rolling forces.
Other drawbacks include inadequate lubrication at the bushing/sleeve interface, and excessive length of the bearing.